Blow molded articles or containers made of thermoplastic materials have been used to package a wide variety of consumer products, such as cosmetics, shampoo, laundry, and food. There are multiple requirements for such containers, particularly if they are required to connote a feeling of quality and prestige to consumers. For example, it is generally desirable for such containers to have a smooth exterior surface to enhance visual effects, such as high glossiness and refinement in dullness. The exterior surface of a container is predominantly dictated by the impression of an inner surface of a mold used in shaping the molded container during a blow molding process. To ensure a smooth exterior surface, it is necessary to use a mold with a smooth inner surface. However, this generally poses other challenges, such as compromising ventilation during the blow molding process thus leading to venting lines and wrinkles in the finished article.
Ventilation allows air between the parison (or preform) of thermoplastic and the mold to escape when the plastic expands into the mold cavity. Without this ventilation, trapped air prevents the thermoplastic material from fully contacting the mold, thus leading to unsightly deformations and e.g., venting lines in the final article. Furthermore, poor ventilation is likely to lead to significant temperature build-up in the mold cavity, which can cause other issues, e.g., adhesion of the thermoplastic to the mold, or unsightly burn marks (i.e., small dark brown or black discolorations on the molded container due to excess heats). These issues are particularly problematic for thermoplastic materials like polyethylene (PE) or polypropylene (PP) because such materials generally have a lower melting point and are accordingly more likely to stick onto a mold at traditional processing temperatures (as compared to materials like polyethylene terephthalate (PET)).
Various solutions have been proposed in the art. For example, in some cases, molds with rough interior surfaces are provided, incorporating micropores made by sandblasting to allow air to migrate through the micropores to the mold vents as the thermoplastic inflates. This reduces the air pressure between the expanding thermoplastic and the mold and mitigates temperature build-up. However, use of a rough mold provides a rough exterior surface for the finished article which is undesirable from an aesthetic point of view.
International Publication No. WO 2015/021843 A1, by the Applicant, discloses a method of making a container where an additive with low surface tension is combined with the thermoplastic material when forming the parison. The low surface tension of the additive prevents the thermoplastic material from sticking to the interior mold and prevents excessive temperature build up in the mold. Thus, the finished article has a smooth exterior surface and glossy finish.
Use of such an additive may, however, be undesirable for reasons such as cost and recyclability. In this respect, introduction of an additive results in one extra raw material and at least one extra step in the manufacturing process. Furthermore, properties of the additive may change the surface energy of the final article, making other surface decoration (e.g., use of labels) difficult.
Some alternative manufacturing methods have also been proposed to address some of the problems outlined above. For example, JP 2013/248798 A describes an extrusion blow molded article using a resin (HDPE) having two layers, an outer layer having “high” MFI of 2-2.5 (measured at 190° C., 2.16 kg) and an inner layer having “normal” MFI (assumed to be around 0.3). A mirror mold is used at an initial mold temperature of between 110° C. and 130° C. After extrusion of the resin parison into the hot mold, cold air is circulated inside the parison to cool the molded article.
The process described in JP 2013/248798 A, however has an unsatisfactory cycle time (for heating the mold and cooling the molded article) which is undesirable from a manufacturing efficiency point of view, and that leads to unacceptable shrinkage of the molded article. In this respect, the slow cooling time allows for more polymer crystallization of the thermoplastic material, which results in shrinkage.
Alternatively, it is known to form an article having two or three layers using extrusion blow molding. The outermost layer in this case has relatively high MFI, whereas the inner one or two layers have “normal” MFI. In this respect, the outermost layer is designed to deliver the required glossy appearance together with the high polished mold, whereas the inner layer(s) provide structural melt strength to support the parison formation during the blowing process.
The article is formed using a high polished mold at a standard EBM molding temperature (approx 20° C.). However, the finished molded article is dull (i.e., not glossy) and features venting lines as air becomes trapped between the parison and the mold.
Thus, there is still a need to provide an alternative manufacturing method that addresses some of the problems still being encountered in the art.